。
She is very fond of that beautiful pearl necklace.
Literal
She [topic-は] that beautiful pearl [genitive-の] necklace [subject-が] greatly is-pleased-with.
気に入っている is the stative form of the idiom 気に入る ('to like, take a liking to') — the ~ている here doesn't mean 'is liking' as an action but 'is in the state of having taken a liking,' i.e., 'is fond of.' 大変 ('greatly, very') is more formal than とても, with a written or polite-conversation flavor. 真珠 ('pearl') is one of Japan's signature exports — Mikimoto Kōkichi pioneered cultured pearl farming in the 1890s in Mie Prefecture, transforming pearls from rare ocean treasures into something widely accessible.